1. Field of the Invention
The present invention relates to a molding method for an optical element and an optical element molding apparatus, in which an optical element is molded by pressing a heated glass material between a pair of dies.
2. Description of the Related Art
Conventionally, as a method of providing an optical element such as a lens by pressing a glass material, there has been generally adopted molding with use of a set of dies constituted by a pair of upper and lower dies and a guide member for holding the upper and lower dies in a sliding manner. In the case of a molding apparatus in which the set of dies constituted by the pair of upper and lower dies and the guide member for holding the upper and lower dies in a sliding manner are used, the guide member has a ring-like shape, and the upper and lower dies are slidably incorporated in a hole of a central portion formed in the guide member. In this case, when a lens is molded, it is necessary to perform molding while preventing relative displacement and inclination between an optical axis of an optical functional surface on one side and an optical axis of an optical functional surface on the other side. For that purpose, not only the upper and lower dies and the guide member has been finished with high accuracy, but also incorporating accuracy between the guide member and the upper and lower dies has been enhanced.
Further, in order to enhance incorporating accuracy, there has been made an attempt to obtain an optical element with excellent in optical axis accuracy by interposing a bearing between the guide member and the upper and lower dies so as to eliminate gaps between the dies and the guide member, which are formed at the time of moving the dies.
Japanese Utility Model Application Laid-Open No. S63-140035 discloses a structure in which the upper and lower die members are guided by the cylindrical member through the intermediary of the ball bearing. Further, there is also disclosed a configuration of setting the coefficient of thermal expansion of the ball bearing to be larger than the coefficient of thermal expansion of the upper and lower die members. With this, under low temperature, the gaps between the upper and lower dies and the ball bearing become larger, thereby facilitating assembly of the dies. Further, under the temperature of press-molding, the gaps between the upper and lower die members and the ball bearing become smaller, thereby enabling manufacture of an optical element of high accuracy, which involves less axial displacement.
However, in recent years, in the field of molding of an optical element with use of glass molding technology, further enhancement in accuracy has been demanded. Simultaneously, further cost reduction has been demanded.
However, in the case of Japanese Utility Model Application Laid-Open No. S63-140035, where the bearing is brought into point-contact with the upper and lower dies and the guide member, it is difficult to achieve the temperate follow-up of the bearing with respect to change in temperature of the upper and lower dies and the guide member. Therefore, there has been a problem in that the temperature of the bearing at the time of heating and cooling and the temperatures of the guide member and the upper and lower dies are liable to be different from each other.
That is, in the case of heating the upper and lower dies, the heating of the bearing is delayed with respect to the heating of the upper and lower dies. Therefore, the temperature of the bearing at the time of heating is lower than the temperatures of the upper and lower dies, and hence the gaps are formed between the upper and lower dies and the guide member. In contrast, cooling of the bearing is delayed at the time of cooling of the upper and lower dies, and the temperature of the bearing is increased in the middle of the cooling. Thus, relative dimensional changes are increased, whereby the gaps with respect to the upper and lower dies and the guide member become smaller so that an excessive compressive force is liable to be generated. Further, in the case of increasing the heating and cooling speeds, the tendency becomes more remarkable.
Further, even when the gaps between the upper and lower dies and the guide member are set in advance to be eliminated at the time of press-molding, the gaps become larger according to the progress of cooling, and hence gaps are formed in the vicinity of a glass transition point. In the vicinity of a glass transition point, the glass is deformable, and hence axial displacement occurs owing to pressure at the time of cooling.